Conventional physical security systems typically employ a breakwire system for detecting entry through the boundaries of a secured area or volume. A breakwire system consists of thin wire routed in serpentine fashion over the boundary surfaces of the area or volume to be protected. Current is passed through the wire in a continuous manner so that any penetration of the boundary surface will break the wire and interrupt the current flow. The interrupted current flow is detected by electronic circuitry which sounds an alarm. One disadvantage of a breakwire system is that to protect the secured area from very small penetrations, such as small diameter drilling, smaller diameter wire must be employed and routed with closer spacing. This significantly increases the cost of the system. In addition, the requirement for an external power source or battery is a disadvantage when the system is to be employed to surreptitiously detect penetrations of a portable object, such as a carrying case.
U.S. Pat. No. 4,954,811, issued Sep. 4, 1990, discloses a penetration sensor employing piezoelectric film. Transducers employing materials having both piezoelectric and pyroelectric characteristics, such as poled polyvinylidene fluoride films, are capable of detecting both temperature changes and vibrations within a wall. The signal produced by a stimulated transducer is supplied to a signal processor which, based on the generated waveform, recognizes the detected activity. Thus, if the signal corresponds to a single impact, such as a wind-blown object, an alarm signal would not be generated. However, if the generated waveform indicates a sudden increase of temperature, such as a fire or an attempted break-in using a torch, an alarm signal would be generated by the system. A system of the type disclosed in U.S. Pat. No. 4,954,811 would not, however, be applicable to a carrying case. Moreover, such a system does not efficiently record penetrations without a battery or external power source.
One goal of the present invention is to provide a sensing/recording system that does not require a battery or external power source to operate. A further goal of the invention is to provide a penetration detection system that may be employed in a "black box" enclosure, e.g., a carrying case, to detect and record penetration of the enclosure.